1. Field of the Invention
The present invention relates to an image reading apparatus that scans an original document and reads the image thereof, and more particularly to an illuminating device that illuminates the original document. It also relates to a light source that is used in such image reading apparatus.
2. Description of the Related Art
FIG. 8 shows a cross-sectional side elevation of a conventional image reading apparatus that scans an original document and reads the image thereof. The image reading apparatus 1 comprises a main body 2 and a cover 3 that is pivotally supported to the main body 2, such that the cover 3 can be rotated up from the right-hand side thereof in the figure. A conveyance unit 5 that conveys the original document sheet W using multiple rollers 5a is disposed at one end of the cover 3.
A glass plate 14 is disposed on the surface of the cover 3 located below the conveyance unit 5 such that the original document sheet W can be conveyed while remaining in contact with the glass plate 14. A paper supply tray 4, in which are housed the sheets of the original document W that are supplied to the conveyance unit 5, is disposed adjacent to the conveyance unit 5. It is also possible to rotate the cover 3 up and place an original document sheet W on the top surface 2a of the main body 2.
Inside the main body 2 are disposed moving units 6 and 7 that can move rightward and leftward (hereinafter the path along which they travel is termed the ‘scanning path’) in the figure. FIG. 9 shows the details of the moving units 6 and 7 as well as the conveyance unit 5. A light source 8 that emits illuminating light from an angled direction relative to the original document sheet W placed on the glass plate 14 is disposed inside the moving unit 6.
The light source 8 is formed in the form of a cylinder that extends perpendicular to the scanning path. FIG. 10 shows a cross-sectional view of the light source 8. The light source 8 comprises a cylindrical glass tube 18 having a fluorescent body 23 applied to the inner wall thereof, and electrodes 21 and 22 mounted to the external circumferential wall thereof. The glass tube 18 is filled with a rare gas 24 such as xenon gas.
The glass tube 18 has a non-applied area 18a on which the fluorescent body 23 is not applied. Because the non-applied area 18a transmits light, this part comprises a light transmission area 8a. When a voltage is impressed between the electrodes 21 and 22, the fluorescent body 23 generates light, which is then emitted via the light transmission area 8a as illuminating light.
In FIGS. 8 and 9, a reflection mirror 9 that reflects the light emitted by the light source 8 and reflected from the original document sheet W is disposed inside the moving unit 6. Reflection mirrors 10 and 11 that receive the reflected light from the reflection mirror 9 and lead it along the scanning path are disposed inside the moving unit 7. An image forming lens 12 and reading unit 13 are disposed in the lower part of the main body 2 such that they face the reflection mirror 11. The image forming lens 12 receives the light reflected by the reflection mirror 11 and causes it to form an image. The reading unit 13 comprises a CCD, which receives the image formed by the image forming lens 12 and converts it into electrical signals.
In the image reading apparatus 1 having the construction described above, when an original document W is placed in the paper supply tray 4 and the conveyance unit 5 is driven, a voltage is impressed between the electrodes 21 and 22 of the light source 8 with the moving units 6 and 7 positioned as shown in FIG. 8. In the light source 8, the fluorescent body 23 generates light and illuminating light is emitted via the light transmission area 8a. 
The illuminating light illuminates the original document sheet W that is conveyed at a prescribed speed by the conveyance unit 5, and the light reflected from the original document sheet is further reflected by the reflection mirrors 9, 10 and 11, whereby it is caused by the image forming lens 12 to form an image on the reading unit 13. Consequently, the reading unit 13 reads the image of the original document sheet, and electrical signals corresponding thereto are output from the reading unit 13.
When an original document sheet W is placed on the top surface 2a of the main body 2, the moving units 6 and 7 move at a prescribed speed along the scanning path. As a result, as the moving units 6 and 7 move, the reflection mirrors 9, 10 and 11 move to positions 9′, 10′ and 11′, respectively, scanning the original document sheet W, and the image thereof is read by the reading unit 13.
However, according to the conventional image reading apparatus 1 described above, in order for the light reflected from the original document sheet W to be received by the reading unit 13, the light source 8 must be retracted from the path of the reflected light. Accordingly, because the light source 8 emits illuminating light onto the original document sheet W from an angle, the distance L2 (see FIG. 9) between the light source 8 and the original document sheet W is long, reducing the illumination efficiency. Therefore, in order to ensure that an amount of light necessary for image reading is emitted, the voltage impressed between the electrodes 21 and 22 must be large, leading to such problems as increased power consumption and thermal deformation of the moving unit 6.